I would say for a max budget of £500 the OP would be looking DIY rather than approved route (or even good co non-approved for that matter) Texe is a well known established brand ( I dont used Texe but thats more by choice than having anything against them) To find a system with all the features of remote arm disarm GSM and other bells and whistle's your choices are rather limited I would be looking for more than twice your budget to supply and install such a system (Mind you I would be installing door contacts at least on your entry route)
So my advise would be:
Texecom is fine, fit a door contact on the entry exit point, read the instructions thoroughly, dont expect to get too many bells and whistles for £500, there are aps available but most go via a cloud, so would require some installer intervention ( Manufacturers do not deal directly with the end user they dont buy enough kit to pay for the support required. Yale etc will deal with GP and are set up to do so) you can get a gsm dialer, but if you use a PAYG sim dont forget to regularly top it up otherwise it wont work when you need it too.